The present invention relates generally to distance measurement systems, and more particularly, to systems for measuring the distance between two objects under dynamic conditions.
The prior art has established numerous techniques for measuring the distance between two objects.
One technique commonly employed to measure the distance between two object involves the use of acoustic and/or ultrasonic waves. For example, in U.S. Pat. No. 4,831,604 to McKnight et al, there is disclosed equipment for ultrasonic range finding. The range finding equipment is employed in a nuclear reactor to monitor the spacing between structural components of the reactor which are under stress. A manipulator carries a pair of send-receive ultrasonic transducers arranged back to back so as to direct ultrasound signals towards reflectors associated with the structural components to be monitored. The transducers are pulsed with signals derived by gating a few cycles of a sustained reference signal of sine wave form and the resulting echo signals can be used to provide transit time and phase displacement information from which the spacing between the reflectors can be derived with a high degree of precision.
The use of ultrasonic and/or acoustic waves in measuring relatively short distances (those distances which are approximately less than 300 feet) between two static objects is an extremely accurate and inexpensive technique as compared to alternative techniques. However, due the relatively slow speed at which ultrasonic/acoustic waves travel, as well as the lack of intensity of said waves, the use of acoustic and ultrasonic waves has been found to be ineffective when used to measure substantial distances between two objects under dynamic conditions. Specifically, because the speed of sound waves is relatively slow, the transit time required for an acoustic or ultrasonic wave to travel from an object to a target and back to the primary object can become quite lengthy when used to measure substantial distances. As a consequence, if the objects to be measured are in motion, it is conceivable that the distance between the objects could change considerably during the measurement period. The movement of the objects during the measurement period will result in an inaccurate measurement of the distance between the two objects (the resulting measurement hereinafter being referred to as an apparent distance measurement between the two objects).
Another technique employed to measure the distance between two objects involves the use of radar, and particularly Doppler radar. Due to the extremely high speed and intensity of radar waves, the use of radar has been found to be extremely accurate and effective in measuring large distances between two objects (e.g. 1000 feet or more). However, for the measurement of short distances, radar has been found to be unreliable and extremely expensive, particularly when compared to the use of acoustic and/or ultrasonic rangefinders.
It should be noted, however, that radar is extremely effective in measuring the relative velocity between two objects at short distances. As a result, radar has been used in the art to accommodate for the movement of the objects during the measurement period of acoustic or ultrasonic rangefinders. The calculation of the relative velocity between objects using radar has been used to determine the distance traveled by the target and/or source during an acoustic or ultrasonic measurement period, which in turn is used to convert the apparent distance between two objects into the true distance between the two objects.
As an example, in U.S. Pat. No. 5,206,652 to Hoyt et al, there is disclosed a height measurement system which uses an inexpensive ultrasonic device to provide an apparent height of a descending airborne object. To compensate for movement of the object during the ultrasonic measurement, a Doppler radar velocity measuring device determines the vertical velocity in a short measurement period. The measured vertical velocity is used to determine the vertical distance travelled during the ultrasonic measurement and to compensate therefor by converting the apparent height to a true height of the descending object. Updates of the true height may be obtained by storing the vertical velocity and retrieving the vertical velocity for multiplication by a time value to obtain an updated value of the true height.
It should be noted that although quite effective, the implementation of radar in the measurement of relatively short distances between objects under dynamic conditions can make the system very costly.